Critical Rod/stroke Ratios?

[B1ack_Mi16 67]

I'm just playing with the thought of a 2.4 litre (2396cc) Mi16 engine as I find my 1.9 DFW to be very slow and sluggish in the 405. Using the 96mm 2.2 crankshaft offsetground to 98.5mm combined with a 88mm bore would make that displacement possible.

 

But the problem I guess is reability due to a Rod / Stroke ratio on the little low side?

 

If I can get pistons with 30mm pin-height / compression height I can use 155,5mm long rods.

 

Browsing Wiseco catalogue I see some pistons are 26.75mm comp-height 86.5mm bore ones, so maybe lets say 27mm comp-height is possible.

 

With 27mm comp-height I can use 158.5mm rods.

 

30mm comp ht gives Rod/stroke: 1.579

27mm comp ht gives Rod/stroke: 1.609

 

I've heard earlier the Danish guys running 2.4 conversions are using a 100mm crank and they apparently gives a bit much sideloadings on the cylinder-wall (they might be using the std. 152mm rods though getting even worse rod/stroke).

 

Standard Mi16 Rod/stroke is 1.625, so I guess the 27mm comp-ht. pistons may work, you think 1.579 Rod/Stroke is engough too?

 

Goal is to make a realible engine.

 

Any thoughts on this, which rod/stroke ratios are OK?

[PumaRacing 2]

Plenty of engines run ratios down into the mid 1.4s without problem. A long stroke Ford Pinto engine has a 127mm rod and 88mm stroke for 1.44:1. They seem happy enough at that.

 

The old Citreon BX14 1361cc engine had a ratio of 1.46 as standard. The 1958cc Honda Accord engine from the 80s was about as bulletproof as anything ever made and ran 142.7mm rods on a 95mm stroke for 1.50:1.

 

Anything close to 1.60 isn't even an issue.

[B1ack_Mi16 67]

That's good to know.

 

Wonder then why they never use 100mm stroke more when building stroked engines in DK.

 

A 100mm stroke and std. 152mm S16 rod would give 1.52, so still should be realible.

 

What other mechanical factors will influence rebility, apart for max-revs of course?

 

Is there maybe a problem using pistons with small deck-height causing them to wear faster?

[TB_205GTI 1]

That's good to know.

 

Wonder then why they never use 100mm stroke more when building stroked engines in DK.

 

A 100mm stroke and std. 152mm S16 rod would give 1.52, so still should be realible.

 

What other mechanical factors will influence rebility, apart for max-revs of course?

 

Is there maybe a problem using pistons with small deck-height causing them to wear faster?

 

exatcly he Pistons Agergard uses are far from perfect, the pistons are too low and the gudgeon pin is placed wrong. The problem with the 2.4's is that the pistons can tilt - thus giving increased bore wear.

[B1ack_Mi16 67]

exatcly he Pistons Agergard uses are far from perfect, the pistons are too low and the gudgeon pin is placed wrong. The problem with the 2.4's is that the pistons can tilt - thus giving increased bore wear.

 

But is a 30mm deck height a problem really?

When wiseco sell them off the shelf, and it will give a reasonable ok rod/stroke with those.

 

I can't see why it shouldn't work.

[sorrentoaddict 1]

Running 1.50-ish Rod/Stroke ratio shouldn't be an

issue for what you are trying to do.

 

In an out-and-out XU race engine, however, when

the aim is to rev around 7-8 krpm "all day", having

in mind the relatively bulky dynamic group of the XU,

1,55-1,60 would already be considered risky.

 

Personally I never wondered why the XU9-J4 posessed

a RL/S ratio of 1,62-something, which is very close to the magic

Phi number - the number of ideal proportion in mechanics and aesthetics...

 

The FIAT 128A.000 engine (a.k.a. FIAT SOHC) has a factory

RL/S ratio of 2,12...

 

It is very much a matter of how dear a metal the factory "affords"

to run for Piston/Liners(Blocks), and whether it is willing to sacrifice

something for engine longevity vs. production margins (the dirty "side"

of the equation that is).

 

On the other hand, the engineering thinking behind some ridicilously

high or ridicolously low RL/S ratios, is often dramatically different in

its roots - for example:

 

in the FIAT case, it must have been either for some ihnerent V.E.

properties that this parameter gives to the engine at certain (high) RPM,

i.e. HOW does it sucks air in (as opposed to HOW MUCH does it suck air in),

or for the vulgar, business-relevant reason mentioned above.

 

in the CIVIC from the early 90's, (much of this goes for the early

ACCORDs too, as they were mentioned here), HONDA used the ridicolously

low RL/S ratio simply because their priority was to design the engine height

as "short" as possible, in order to have a very low hood-height and waistline

of the car, for various aerodynamic/aesthetic/handling/and general "less is more"

weight-biased thinking (and, why not, lower production (material) cost of the car).

 

what REALLY surprises us these days, is HOW BLOODY much the RL/S ratio

differs from one Sports-Bike manufacturer to another, in spite of the fact

that the 600cc Super-Sports bikes seem to be alike in terms of general

engineering "thinking".

 

Btw. do you really think that a 307 is safer than a 205 ?

When you'd (God forbid) had a choice of being hit with

a 307 crossing on "red" with 40 MPH, and a 205 crossing on "red"

with 40 MPH, what do you think your chances would be in both scenarios?

(same goes for a frontal impact, too).

 

The only real reason behind the fact that the avg. family car of the 2006-age

weighs at cca. 1,350 kg., whilst the avg. family car of the 1980's weighed

at cca. 850 kg -- is that some guys out there invoice almost double more steel

per one-car-sold. Someone must have had a great idea how to double their

turnover, convincing us that more is more... Modern cars are actually FAR

less safe - and that can be confirmed by anyone being hit by a car of 1,500kg

at speed.

 

Often it is same with the RL/S ratio and various other "engine" related issues,

such as the 16-valve story etc. etc.. (more valves, more steel, more followers,

more springs = MORE CASH)

 

Sorry for the hijack

 

AG

[PumaRacing 2]

But is a 30mm deck height a problem really?

When wiseco sell them off the shelf, and it will give a reasonable ok rod/stroke with those.

 

I can't see why it shouldn't work.

 

I have a drawing here for the piston from a Yamaha 600cc single 4v bike engine from 1984 on. 95mm bore, 84mm stroke, 28mm deck height and the whole piston is only 58mm long. It looks more like a disc than a piston. Look at that deck height in relation to the bore size. We're also used to pistons being somewhat square, i.e. about the same length as bore size but this thing has a ratio of 1.64. Bloody scary if you ask me but I think Yamaha know what they're doing when it comes to engines.

 

Plenty of production car engines are now running deck heights of less than 30mm. I think the late model Ford Zetec is about 28mm and those things are bulletproof. What you don't want with a short piston is a central gudgeon pin though. That offset of 1mm or so to the thrust side helps counter any rock at TDC.

 

I remember several years ago a race team coming to me for consultation on a piston problem they were having in a VW engine. Custom forged pistons from some yank outfit. Hairy arse I think it was. The things kept seizing and breaking up. The yanks kept telling them to give it more bore clearance. That just made things worse. I had a look and a measure. They were s*ite. The pins were central and the ring land diameter was far too small for the bore so there was no support above the pin even when they got red hot. The things could rock about like a d*ck in a drainpipe. At TDC they banged over violently from one side to the other and then ripped the whole top land off when the crank started back down again. I advised them to use them as ashtrays or paperweights but under no circumstances as pistons. They went back to the OE cast pistons and never had another issue even at 12.6:1 CR.

 

OE manufacturers tend to make very nice pistons and they test numerous designs to get wear, rock, expansion, bore clearance, oil consumption and NVR right. Trouble is some of the aftermarket 'race' piston makers couldn't design their way out of a paper bag. They just make vaguely round things and suggest loads of bore clearance in the hope nothing will seize. If the engine sounds like a spindrier full of bolts then tough. It's a race engine mate, what do you expect?

 

Pistons can be very critical to the rest of the engine design. Cast iron block, wet liner ally block, aluminium bores with Nikasil - it all alters the required skirt shape and bore clearance in various places. Top land clearance can be very tricky and depends how hot the piston crown gets and that's also a function of state of tune and compression ratio. Often you don't get asked any of these questions. You just get the bog standard Mk1 forged piston variant and are told to run it with 4 thou bore clearance. That's a sort of standing joke in piston land. When in doubt slap it in at 4 thou bore clearance. With luck it won't be too tight and seize or loose enough that it knocks itself to bits. Not that you get a warranty anyway. It's a race engine.

 

What always amazes me is the abuse you can throw at bog standard cast OE pistons running three fifths of f*ck all bore clearance as long as you haven't done anything silly like set the mixture up weak or given it so much ignition advance it detonates. You can slap a turbo on, triple the bhp output, even throw in a splash of nitrous and they just soak it all up. With modern casting alloys and steel expansion struts cast in they'll do all that at 4 tenths of a thou bore clearance too. If the block's in a cold workshop and the pistons come out of a warm kitchen you can't even get the buggers in the holes.

[petert 586]

With modern casting alloys and steel expansion struts cast in they'll do all that at 4 tenths of a thou bore clearance too.

 

Even expansion struts are considered "old skool" now. The change from hypo to hyperetectec alloy is the reason for big increase in OEM piston strength and stabilty.

[M3Evo 0]

What do those 'strut' things actually do?

 

Presumably they're there to reduce expansion of the skirt or something what with the steel having a lower coefficient of thermal expansion?